cue: dig up wrapped error in valueError
cue: dig up wrapped error in valueError

Change-Id: Ic65285e5c54b59734da9d9a074416272c26cbe57
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7682
Reviewed-by: CUE cueckoo <0f32a827c9a5f00281f936ac78b5ef3ba80a954c@gmail.com>
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue